How do you apply a gloss map?
A gloss map is a map of specular power, the inverse of a roughness map.  Read it as non-color, then subtract it from 1 (but see below) via a math node and plug it into the roughness value of any/all shaders.

There is no single, standardized format for gloss/roughness/specular power-- there are instead a lot of different ways to map from color to gloss, depending on engine.  So in order to use a gloss map correctly, you have to know the engine for which it was made, and then you have to know how that engine interprets gloss.

Or you could just wing it.  Start with the simple inversion I mentioned above.